Method for the Chip-Integrated Spectroscopic Identification of Solids, Liquids, and Gases

ABSTRACT

Methods and systems for a label-free on-chip optical absorption spectrometer consisting of a photonic crystal slot waveguide are disclosed. The invention comprises an on-chip integrated optical absorption spectroscopy device that combines the slow light effect in photonic crystal waveguide and optical field enhancement in a slot waveguide and enables detection and identification of multiple analytes to be performed simultaneously using optical absorption techniques leading to a device for chemical and biological sensing, trace detection, and identification via unique analyte absorption spectral signatures. Other embodiments are described and claimed.

I. CROSS REFERENCE TO RELATED APPLICATIONS

The application is a continuation application of U.S. patent applicationSer. No. 13/607,794, titled “Fabrication Tolerant Design for theChip-Integrated Spectroscopic Identification of Solids, Liquids, andGases,” filed Sep. 9, 2012, which is a continuation-in-part applicationof U.S. patent application Ser. No. 12/806,840, titled “Photonic CrystalSlot Waveguide Miniature On-Chip Absorption Spectrometer,” filed Aug.23, 2010, now U.S. Pat. No. 8,282,882, issued Oct. 9, 2012, the contentsof which are all hereby incorporated by reference.

II. BACKGROUND

1. Field of the Invention

This invention relates generally to the field of optical and medicaldevices, and more specifically to an apparatus and method for on-chipoptical spectroscopy for the detection and identification by uniquespectral signatures of solid, liquid, or gas substances using photoniccrystals.

2. Background of the Invention

Defect engineered photonic crystals, with sub micron dimensions havedemonstrated high sensitivity to trace volumes of analytes forperforming a large range of sensing applications. Photonic crystals havebeen described and discussed by Joannopoulos, J. D., R. D. Meade, and J.N. Winn, in Photonic Crystals, 1995 Princeton, N.J.: PrincetonUniversity Press. However, exact identification of analyte throughspectroscopic signatures has not been demonstrated. Furthermore, much ofthe research in photonic crystal devices has relied on enhancingrefractive index sensitivity to a single analyte (see Lee M. R., andFauchet M., “Nanoscale microcavity sensor for single particledetection,” Optics Letters 32, 3284 (2007)) and detection of a singleanalyte. Some of the popular commercially available optical spectroscopytechniques are cavity ringdown spectroscopy (CRDS) described by ThorpeM. J. et al., in “Broadband cavity ringdown spectroscopy for sensitiveand rapid molecular detection,” Science 311, 1595 (2006), and tunablediode laser absorption spectroscopy (TDLAS) described by Druy M., in“From laboratory technique to process gas sensor: the maturation oftunable diode laser absorption spectroscopy”, in Spectroscopy 21(3), 14(2006). Cavity ringdown spectroscopy cannot be integrated on-chipprimarily due to the stringent requirement of optical source and highfinesse optical cavities for analyte sampling. Technically, tunablediode laser absorption spectroscopy could be integrated on-chip, but theanalyte sampling volume required is of the order of meters and cannottherefore be integrated on a chip. Hence, tunable diode laser absorptionspectroscopy is also not a convenient method to integrate on-chip.Furthermore, both cavity ringdown spectroscopy, due to the requirementof femtosecond lasers and high finesse optical cavities for highsensitivity, and tunable diode laser absorption spectroscopy, are veryexpensive, of the order of tens of thousands of dollars. A primarydrawback of both cavity ringdown spectroscopy and tunable diode laserabsorption spectroscopy is the weight, size, as well as cost whichsignificantly increases the cost of ownership of the correspondingproducts. The same drawbacks of size, weight, and cost hold for Fouriertransform infrared spectroscopy and photoacoustic spectroscopy. Alab-on-chip integrated infrared spectrometer for remote, in-situ sensingand spectroscopic identification is highly desired for widespreaddeployment to enhance the detection of hazardous pollutants forenvironmental and homeland security. Research on on-chip spectroscopyhas resulted in methods such as a ring resonator based microspectrometerdescribed by Kyotoku B. B. C., Chen L., and Lipson M., in “Sub-nmresolution cavity enhanced microspectrometer,” in Optics Express 18(1),102 (2010) for optical spectral analysis, by Robinson J. T., Chen L.,and Lipson M., in “On-chip gas detection in silicon opticalmicrocavities,” in Optics Express 16(6), 4296 (2008) for gas sensing andphotonic crystal microcavity-based devices described by Canon KabushikiKaisha in US Patent Application 20060285114, “Gas detection and photoniccrystal devices design using predicted spectral responses” which rely onthe quality factor of the optical microcavity to trap light and enhancethe optical interaction with the analyte by increasing the effectiveinteraction time with the analyte with photonic crystal microcavities.However, these devices are limited by resolution of spectrometry. Inorder to increase the resolution of the spectrometer, either the size ofthe ring resonator must be increased, the number of photonic crystalmicrocavities must be increased in proportion to the resolution, orcomplex grating structures need to be fabricated as described by KyotokuB. B. C., Chen L., and Lipson M., in “Sub-nm resolution cavity enhancedmicrospectrometer,” in Optics Express 18(1), 102 (2010). Research hasbeen performed with hollow core waveguides for atomic spectroscopyon-chip by Yang W., Conkey D. B, Wu B., Yin D., Hawkins A. R., andSchmidt H., in “Atomic spectroscopy on a chip” in Nature Photonics 1,331 (2007), which are limited to large sizes when the absorptioncross-section of the analyte becomes small.

Two dimensional photonic crystal waveguides integrated with slotwaveguides offer the possibility of integrating the slow light effect oftwo-dimensional photonic crystal waveguides with the optical fieldenhancement effect of slot waveguides to enhance the optical interactionbetween light and analyte. According to Beer-Lambert absorptiontechnique, transmitted intensity I is given by equation 1 as

I=I ₀exp(−γAL)  (1)

where I₀ is the incident intensity, A is the absorption coefficient ofthe medium, L is the interaction length, and γ is the medium-specificabsorption factor determined by dispersion enhanced light-matterinteraction. In conventional systems, L must be large to achieve asuitable sensitivity of the measured I/I₀. For lab-on-chip systems, Lmust be small, hence γ must be large. Mortensen et al. showed [MortensenN. A., Xiao S. S., “Slow-light enhancement of Beer-Lambert-Bouguerabsorption,” Applied Physics Letters 90 (14), 141108 (2007)] usingperturbation theory that

$\begin{matrix}{\gamma = {f \times \frac{c/n}{v_{g}}}} & (2)\end{matrix}$

where c is the velocity of light in free space, v_(g) is the groupvelocity in the medium of effective index n, and f is the filling factordenoting the relative fraction of the optical field residing in theanalyte medium. Equation 2 shows that slow light propagation (smallv_(g)) significantly enhances absorption. Furthermore, the greater theelectric field overlap with the analyte, the greater the effectiveabsorption by the medium. In a conventional waveguide, the optical modeinteracts with the analyte only through its evanescent tail. In a slotwaveguide, the guided optical mode not only interacts with the analyteenvironment with its evanescent tail, but also interacts with theenhanced optical field intensity in the slot. In a photonic crystalwaveguide as theoretically proposed by Mortensen, only the groupvelocity v_(g) is reduced. By introducing a slot in a two-dimensionalphotonic crystal waveguide, we have demonstrated experimentally that ina two-dimensional photonic crystal slot waveguide, the group velocity isreduced by a factor of 100 due to the slow light effect and the opticalfield intensity is increased by a factor of 10 in a slot compared toevanescent guiding only. As a result, the effective absorption length isincreased by a factor of 1000 compared to the geometrical opticallength, which increases the optical absorption by the analyte, asdetermined by the Beer-Lambert law of optical absorption. The factor of1000 is much larger than the factor of 10 demonstrated by Jensen et al.using a one-dimensional Bragg stack that employs group velocity v_(g)reduction only and has a much smaller slow light effect due toone-dimensional confinement of the slow light propagating optical mode[Jensen K. H., Alam M. N., Scherer B., Lambrecht A., Mortensen N. A.“Slow light enhanced light-matter interactions with applications to gassensing”, Optics Communications 281 (21), 5335 (2008)]. The factor of1000 improvement in our photonic crystal slot waveguide spectrometer dueto the combined effects of high group velocity enhancement and opticalintensity enhancement in a two-dimensional photonic crystal slotwaveguide results in absorption lengths of the order of hundreds ofmicrons on-chip compared to tens of centimeters in single pass off-chipspectroscopy techniques.

Spectrometer techniques such as cavity ring-down spectroscopy, ringresonator optical cavity, and photonic crystal cavity are limited inresolution, by the finesse of the optical cavity and the size and weightof the optical cavity. Designs are needed to make a miniaturized,light-weight on-chip integrated spectrometer that can measure acontinuous absorption spectrum, not limited by resolution of thespectrometer.

Designs are therefore needed in the art to integrate two-dimensionalphotonic crystal waveguides with slot waveguides for optical absorptionspectroscopy on-chip.

III. SUMMARY

One embodiment of the invention provides a spectrometer comprising asemiconductor material core with high dielectric constant, supported onthe bottom by a low dielectric constant cladding. A triangular latticeof photonic crystal holes is etched into the substrate. A photoniccrystal waveguide is defined by filling a single row of holes, from theinput ridge waveguide transition to the output ridge waveguidetransition, with the semiconductor core material. A slot waveguide issimilarly defined by etching a slot in the semiconductor core materialand filling the photonic crystal waveguide. The high dielectric constantcore with structured photonic crystal slot waveguide, together with thelow dielectric constant cladding, form the photonic crystal slotwaveguide structure. Light is coupled into the photonic crystal slotwaveguide from a ridge waveguide. Light is out-coupled from the photoniccrystal slot waveguide to an output ridge waveguide. When a broadbandlight source is input to the photonic crystal slot waveguide, increasedabsorption of light occurs due to the slow light effect of photoniccrystal waveguide and enhanced optical field in the slot. Depending uponthe wavelength range of interrogation, which is determined by theabsorption wavelength of the analyte, the period of the sub-wavelengthlattice can vary from 50 nm to 1500 nm and the depth of the latticestructure can vary from 0.4 to 0.7 times above the lattice periodicity.The semiconductor material can be silicon (or any Group IV material),gallium arsenide (or any III-V semiconductor), or any semiconductormaterial with high refractive index. The substrate can be any Group IVmaterial corresponding to the Group IV core material or any substratesuitable to grow the III-V core material. A single transmission spectrumis measured in the absence of the analyte. A second transmissionspectrum is measured in the presence of the analyte. The differencebetween the first and second transmission spectra determine theabsorbance of the analyte in the interrogated wavelength range. Analyzedanalytes can be pollutant gases such as greenhouse gases (carbondioxide, carbon monoxide, methane, nitrous oxide, chlorofluorocarbons,ozone or volatile organic compound (such as benzene, toluene, xylene,ethylbenzene) vapors), hazardous gases such as ammonia, explosives suchas TNT (trinitrotoluene) or RDX, by-product gases from battery offgassing in lithium ion batteries, sulfur dioxide, automobile engineexhaust, and stack exhausts in coal and oil refineries. The analyte canalso be any volatile organic compounds in liquid form (such as benzene,toluene, xylene, ethylbenzene) dissolved in water. The analyte can alsobe solid particles or biomolecules. The optical absorption can be doneat any visible, near infrared, or far infrared region of theelectromagnetic spectrum, to measure either overtones or fundamentalvibration modes of analyte molecules in solid, liquid, or gas.

To Summarize:

The primary objective of the invention is to provide an on-chipintegrated photonic crystal slot waveguide absorption spectrometer withcompact size that can be monolithically integrated with lasers anddetectors for the spectroscopic identification with unique absorptionsignatures of solids, liquids, and gases.

The second objective of the invention is to enable an on-chip analytesampling cell determined by the slot dimensions in the photonic crystalslot waveguide, which enhances the optical absorption between theanalyte and light over a given geometrical length on a chip, therebyeliminating the need for multiple pass absorption cells and complexoptical elements employed in off-chip analyte sampling cells.

The third objective of the invention is to significantly increase thenumber of analytes that can be measured on an integrated chip level.Contemporary systems for measuring gases by cavity ring-downspectroscopy can measure up to 16 gases. However, the correspondingequipment can weigh approximately 100 pounds and occupy about 3 cubicfeet of space. Since the present invention considers on-chip integrationof photonic crystal slot waveguides that occupy less than 0.2 mm² each,dense integration of waveguides can cover the entire wavelength spectrumfrom the near-infrared to the far-infrared on the same silicon chip.

The fourth objective of the invention is to implement a novelspectrometer scheme on a CMOS chip that provides a continuous spectrum,where the spectrometer itself minus the detector, is not limited byresolution, as is the case for all on-chip and off-chip spectrometers.

Other objectives and advantages of the present invention will becomeapparent from the following descriptions, taken in connection with theaccompanying drawings, wherein, by way of illustration and example, anembodiment of the present invention is disclosed.

IV. BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and includeexemplary embodiments of the present invention, which may be embodied invarious forms. It is to be understood that in some instances variousaspects of the present invention may be shown exaggerated or enlarged tofacilitate an understanding of the invention.

A more complete and thorough understanding of the present invention andbenefits thereof may be acquired by referring to the followingdescription together with the accompanying drawings, in which likereference numbers indicate like features, and wherein:

FIG. 1 is a top view schematic drawing showing the design of a photoniccrystal slot waveguide based on a slot integrated with a photoniccrystal waveguide.

FIG. 2 is a cross-sectional view of the device shown in FIG. 1 takenalong line A-A′.

FIG. 3 is a top view of the field intensity pattern of a guided mode ofa photonic crystal slot waveguide depicted in FIG. 1 and FIG. 2.

FIGS. 4 a and 4 b illustrate typical diagrams of the dispersion relationof a photonic crystal slot waveguide in the low group velocity region.

FIG. 4 c illustrates a possible normalized dispersion diagram of thephotonic crystal slot waveguide.

FIG. 4 d is a top view schematic drawing showing the design of afabrication tolerant photonic crystal slot waveguide.

FIG. 4 e illustrates the fabrication tolerant design concept plottingthe group index calculated from the dispersion diagram of photoniccrystal slot waveguide, to ensure a specific absorbance efficiency bythe analyte.

FIG. 4 f illustrates the group index as a function of wavelength for aseries of group index plots.

FIG. 4 g illustrates a typical design of the input impedance taper inthe fabrication tolerant design which achieves the gradual change ingroup index in FIG. 4 f.

FIG. 5 a shows the experimental transmission spectrum of a functionalphotonic crystal slot waveguide in the absence of methane as arepresentative analyte, as a function of wavelength.

FIG. 5 b shows the experimental transmission spectrum of a functionalphotonic crystal slot waveguide in the presence of methane as arepresentative analyte, as a function of wavelength.

FIG. 5 c shows the difference spectrum which determines the absorbancecharacteristics of the analyte, in this case, methane gas as a functionof wavelength.

V. DETAILED DESCRIPTION Detailed Description of the Invention

In accordance with a preferred embodiment of the present invention, adevice for an on-chip integrated optical absorption spectroscopycomprises: a functional photonic crystal waveguide having a waveguidecore along which light is guided, a slot at the center of the photoniccrystal waveguide along the length of the photonic crystal waveguide, aninput and output photonic crystal waveguide with gradually changed groupindex before and after the functional photonic crystal waveguide, whichcan bridge the refractive indices difference between conventionaloptical waveguides and the functional photonic crystal waveguide. Thesensor can be used to detect organic or inorganic substances that can besolids and liquids. Light (from a broadband source or LED) coupled intoa photonic crystal slot waveguide, in the presence of the analyte, hasenhanced absorption by the analyte due to the increase in the effectiveoptical path length caused by the enhanced field intensity in the slotand the slowdown effect of photonic crystal waveguide dispersion.Transmission spectra are measured covering the entire transmissionbandwidth of the photonic crystal slot waveguide, both in the presenceand absence of the analyte. The presence of the analyte leads to adecrease in transmission intensity due to absorption, compared to thetransmission in the absence of the analyte. Absorbance spectrum of theanalyte is determined from the difference in transmission.

In another embodiment of the present invention, a device for an on-chipintegrated optical absorption spectroscopy comprises: a functionalphotonic crystal waveguide having a waveguide core along which light isguided, a slot at the center of the photonic crystal waveguide along thelength of the photonic crystal waveguide, an input and output photoniccrystal waveguide with gradually changed group index before and afterthe functional photonic crystal waveguide, which can bridge therefractive indices difference between conventional optical waveguidesand the functional photonic crystal waveguide. The top cladding is airin which the analyte of interest must be detected. The sensor can beused to detect presence of pollutant gases such as greenhouse gases(carbon dioxide, carbon monoxide, methane, nitrous oxide,chlorofluorocarbons, ozone or volatile organic compound (such asbenzene, toluene, xylene, ethylbenzene) vapors), hazardous gases such asammonia, explosives such as TNT (trinitrotoluene) or RDX, by-productgases from battery off gassing in lithium ion batteries, sulfur dioxide,automobile engine exhaust, stack exhausts in coal and oil refineries.Light (from a broadband source or LED) coupled into a photonic crystalslot waveguide, in the presence of the analyte, has enhanced absorptionby the analyte due to the increase in the effective optical path lengthcaused by the enhanced field intensity in the slot and the slowdowneffect of photonic crystal waveguide dispersion. Transmission spectraare measured covering the entire transmission bandwidth of the photoniccrystal slot waveguide, both in the presence and absence of the analyte.The presence of the analyte leads to a decrease in transmissionintensity due to absorption, compared to the transmission in the absenceof the analyte. Absorbance spectrum of the analyte is determined fromthe difference in transmission.

In another embodiment of the present invention, a device for an on-chipintegrated optical absorption spectroscopy comprises: a functionalphotonic crystal waveguide having a waveguide core along which light isguided, a slot at the center of the photonic crystal waveguide along thelength of the photonic crystal waveguide, an input and output photoniccrystal waveguide with gradually changed group index before and afterthe functional photonic crystal waveguide, which can bridge therefractive indices difference between conventional optical waveguidesand the functional photonic crystal waveguide. The top cladding is alayer of organic polymer such as PDMS (poly-dimethyl-siloxane) or PMMA(poly-methyl methyl-acrylate) that is hydrophobic but readily swells inthe presence of volatile organic compounds such as benzene, toluene,xylene, or ethylbenzene. The polymer which forms the top cladding alsofills the photonic crystal holes as well as the slot in the middle ofthe photonic crystal slot waveguide. Light (from a broadband source orLED) coupled into a photonic crystal slot waveguide, in the presence ofthe analyte, has enhanced absorption by the analyte due to the increasein the effective optical path length caused by the enhanced fieldintensity in the slot and the slowdown effect of photonic crystalwaveguide dispersion. Due to the water filtering capability of thehydrophobic polymer, only the volatile organic compound contaminants inthe water are absorbed by the polymer; light is guided in the photoniccrystal slot waveguide and transmission spectra are measured withoutinterference from the strong absorption signatures of water.Transmission spectra are measured covering the entire transmissionbandwidth of the photonic crystal slot waveguide, both in the presenceand absence of the analyte, in this case the volatile organic compounds,in the water. The presence of the analyte leads to a decrease intransmission intensity due to absorption, compared to the transmissionin the absence of the analyte. Absorbance spectrum of the analyte isdetermined from the difference in transmission, without interference ofthe water medium in which the analyte of interest is located.

For the measurement of environmental parameters in situ, the device isincorporated with a filter to remove macroscopic dirt and dust particlesand moisture. The filter can be a macroscopic filter incorporatedoff-chip or a microfluidic filter incorporated on-chip.

For the measurement of environmental parameters in-situ from remotedistances, the device is incorporated with an optical fiber to couplelight into and out of the chip. The spectrometer can also bemonolithically integrated with semiconductor chip based lasers anddetectors, that emit and detect at the absorbance wavelengths ofinterest of the target gas, and the device can be operated usingwireless signals. No line-of-sight based limitations exist for theinvention for measuring at remote distances in-situ in tight spaces,such as those in quantum cascade laser based open-path andretro-reflector systems.

For the measurement of different substances that have absorbancecharacteristics in different regions of the electromagnetic spectrum,due to the scalability of Maxwell's equations, the device can simply bescaled geometrically in proportion to wavelength without altering theratio of semiconductor core material and empty space, called the fillingfraction of the semiconductor core material. The filter can be amacroscopic filter incorporated off-chip or a microfluidic filterincorporated on-chip.

Methods for fabricating photonic crystal slot waveguide structures arewidely described in the literature. The amount of absorption and hencethe change in transmission intensity is determined by the concentrationof analyte, the absorption cross-section of analyte, the wavelength ofinterrogation (whether near-infrared or far-infrared), the harmonic thatis interrogated (whether fundamental vibrations or overtones) and thegeometrical length of the photonic crystal slot waveguide.

On-chip optical spectrometers provide an unprecedented opportunity forcomprehensive concurrent analysis of thousands of analytes such ashazardous gases and pollutants, greenhouse gases, explosive gases,biomolecules, contaminant solids, and liquids. The spectroscopicanalysis response to a toxic agent, provides a more unique signature ofthe toxicologically significant substance. The spectrometers haveutility in the fields of industrial process control and monitoring,homeland security, environmental protection, and bio- and chemicalwarfare defense.

The principles of the invention can also be applied to tunable diodelaser absorption spectroscopy incorporating a photonic crystal slotbased optical waveguide. The enhanced optical absorption enabled by slowlight and slot enhancement, when combined with the principles of tunablediode laser absorption spectroscopy using either wavelength modulationor frequency modulation, will enable on-chip optical absorptionspectroscopy, for highly sensitive lab-on-chip spectroscopy. In suchcases, the tunable laser/light emitter wavelength that coincides withthe absorption signature of the analyte of interest is guided over thebandwidth of the slow light region in the photonic crystal slotwaveguide and the analyte sampled by the slot in the middle of thephotonic crystal slot waveguide.

Detailed descriptions of the preferred embodiments are provided herein.It is to be understood, however, that the present invention may beembodied in various forms. Therefore, specific details disclosed hereinare not to be interpreted as limiting, but rather as a basis for theclaims and as representative basis for teaching one skilled in the artto employ the present invention in virtually any appropriately detailedsystem, structure or manner.

A lab-on-chip integrated infrared spectrometer for remote, in-situsensing and spectroscopic identification of hazardous pollutants in theenvironment, specifically greenhouse gases, is highly desired. Defectengineered photonic crystals, with sub micron dimensions have alreadydemonstrated high sensitivity to trace volumes of analytes. However,exact identification of analyte through spectroscopic signatures has notbeen demonstrated. Our photonic crystal slot waveguide device combinesthe slow light effect in photonic crystal waveguides with large opticalfield intensity in a low index slot at the center of the photoniccrystal waveguide to enhance interaction between the optical field andthe analyte gas. Transmission spectrum of photonic crystal slotwaveguide is measured in presence and absence of methane gas andabsorbance determined from difference in transmission. The waveguide isfabricated on a silicon-on-insulator wafer with standard complementarymetal oxide semiconductor (CMOS) fabrication steps. Our generic on-chipspectrometer will enable low cost on-chip chemical and biologicalanalysis and diverse applications in industrial process control andmonitoring. This section will provide detailed description of thepreferred embodiments in the aspect of device architecture, as well asthe design, concept, and working principle.

FIG. 1 presents a top view schematic drawing of a photonic crystal slotwaveguide. It consists of a functional photonic crystal waveguide 100,input ridge waveguide 112, output ridge waveguide 113, and a slot 101extending from input to output. The combination of photonic crystalwaveguide 100 and slot 101 is designated as the functional photoniccrystal slot waveguide 102. The functional photonic crystal slotwaveguide 102 includes a number of column members 103 etched through orpartially into the semiconductor slab 104. The waveguide core 141 isdefined as the space between the centers of two column members adjacentto the region where the columns are absent. In one preferred embodiment,the column members 103 are arranged to form a periodic lattice with alattice constant α. In some embodiments, the width of waveguide core 141can range from 0.5 times sqrt(3) times the lattice constant or period ato 50 times sqrt(3) times the lattice constant or period α. The arrowsindicate the direction in which electromagnetic waves are coupled intoand out of the photonic crystal waveguide respectively. The photoniccrystal waveguide 100 is defined by filling a complete row of columnarmembers with the semiconductor slab material 104. The slot 101 is etchedinto the photonic crystal waveguide 100. The width of the slot 101 canrange from an infinitesimally narrow width of 10 nanometers or smalleras limited by any lithography method to as large as the width of thewaveguide core 141. The number of slots 101 can vary from one (1) toinfinity, solely determined by the number that can be accommodated inthe width of the photonic crystal waveguide 100, thereby creating eithera photonic crystal single slot or multiple slot waveguide. Withreference to FIG. 2, which is a cross-sectional view of the functionalphotonic crystal slot waveguide 102 in FIG. 1 taken along line A-A′, thecolumn members 103 extend throughout the thickness of the slab 104 toreach a bottom cladding 105. In one embodiment, the top cladding 106 isair. The material of the top cladding also fills the column members 103and the slot 101 in the middle of the photonic crystal slot waveguide102. However, the material which constitutes the top cladding 106 andfills the column members 103 and slot 101, can be any hydrophobicorganic material such as PDMS (poly-dimetyl siloxane) or PMMA(poly-methyl methylacrylate) that absorbs and swells in the presence oforganic contaminants in water and at the same time, does not absorbwater. The material which constitutes the top cladding 106 and fills thecolumnar members 103 and slot 101 can be any other organic or inorganicdielectric material that suitably isolates the analyte whose absorptionspectrum is to be measured, from the background medium, when theabsorption spectrum of the analyte to be measured is small and can bemasked by the overlapping absorption signatures of the backgroundmedium. Columnar members 103 can extend through both 106 and 104 as wellas through the bottom cladding 105 to reach the substrate 107. The topcladding 106 can extend either finitely or infinitely in the verticaldirection, perpendicular to the slab 104. When the top cladding 106extends finitely, and it is desired to obtain the analyte spectrumindependent of the background medium, the thickness of the top cladding106 must be sufficient so that the tail of optical mode which propagatesdown the photonic crystal slot waveguide does not interact with thebackground medium. Although the structure within the slab 104 issubstantially uniform in the vertical direction in this embodiment,vertically non-uniform structure, such as the columnar members 103 whoseradii are varying along the vertical direction, may be used as well. Thecolumn members 103 can be either simply void or filled with otherdielectric materials. Between the ridge waveguide 112 and the corephotonic crystal slot waveguide 102, there is an impedance taper 115 forcoupling of light from the ridge waveguide to the photonic crystal slotwaveguide with high efficiency. Similarly, at the output, between thephotonic crystal slot waveguide 102 and the output ridge waveguide 113,there is another impedance taper 114 for better coupling efficiency. Theimpedance taper 115 is formed by shifting the columnar members fromphotonic crystal waveguide 100 to ridge waveguide 112 by x times α inthe direction perpendicular to 100, in the plane of the waveguide, insteps of (x times α)/p where p is a number greater than 4, α is thelattice constant, and x varies from 0 to z, where z is a fractionalnumber between zero and one (0<z<1). In FIG. 1, p equals six since it isthe first six columnar members that have been shifted in both of theimpedance tapers 115 and 114. In some embodiments, the impedance tapers115 and 114 may be oppositely tapered. In that embodiment, impedancetaper 115 would be narrower at the ridge waveguide 112 than at thephotonic crystal slot waveguide 102 and impedance taper 114 would benarrower at the ridge waveguide 113 than at the photonic crystal slotwaveguide 102. In this embodiment, x varies from 0 to z, where z is afractional number between zero and minus one (−1<z<0). For a photoniccrystal waveguide 100, 114 and 115, which comprise photonic crystals oftwo-dimensional periodicity, the wave guiding in the vertical directionmust be provided by conventional index-guiding scheme. This means abottom cladding 105 and a superstrate 106 with a lower effective indexrelative to that of the slab material must be disposed below and abovethe slab 104. In FIG. 2, the superstrate is 106 and simply representedby air or vacuum. The superstrate 106 can also be any organic orinorganic material. On one side, bottom cladding 105 and superstrate 106prevent guided lightwave escaping far away from the top and bottomsurfaces of the slab 104. In most applications, it is desirable that thewaveguide have a single guided mode, which can be achieved throughadjusting the width of waveguide core 141.

FIG. 3 depicts a top view of the field intensity pattern of a guidedmode of a photonic crystal slot waveguide 102 in FIG. 1 and FIG. 2. Thecircles indicate columnar members 103 of the photonic crystal waveguide.It is seen in FIG. 3 that peak of the field intensity indicated by thehatched regions within the slot is well confined inside the slot 101 ofthe photonic crystal slot waveguide 102 in the core region 141.

The bold curve 190 in FIG. 4 a depicts a dispersion diagram of thephotonic crystal slot waveguide 102. Based on design requirements, thedispersion curve 190 can be shifted up or down in frequency, as shown inFIG. 4 a, by adjusting the width of the photonic crystal waveguideregion 100 or the width of the slot 101 constituting the photoniccrystal slot waveguide 102. The photonic crystal slot waveguide mode inthe waveguide core region 141 is confined in the photonic band gap 161,the region between the air band 151 and dielectric bands 152 as shown inFIG. 4 a. The width of the slot 101 can range from an infinitesimallynarrow width of 10 nanometers or smaller as limited by any lithographymethod to as large as the width of the waveguide core 141, that givesrise to a photonic crystal slot waveguide mode confined in the photonicband gap 161, the region between the air band 151 and dielectric bands152 as shown in FIG. 4 a. FIG. 4 b shows one of the curves 190 for awaveguide with x equal to 0.9. Two regions are distinctly visible in thebold curve, a region 163 where d∩/dk is high and a region 162 whered∩/dk is low. d∩/dk denotes the group velocity of light propagating inthe photonic crystal waveguide at the corresponding frequency. Region162 is the design frequency range in a tunable diode laser absorptionspectroscopy, tunable laser absorption spectroscopy or tunable lightemitter absorption spectroscopy on-chip device. The enhancement ofoptical absorption is inversely proportional to the group velocity,consequently the slower the group velocities, the higher the enhancementof optical absorption by the analyte filling the slot 101 in thephotonic crystal slot waveguide 102.

FIG. 4 c represents another possible normalized dispersion diagram ofthe photonic crystal slot waveguide. The dispersion diagram isnormalized to the lattice constant. Hence, by choosing an appropriatelattice, α, one skilled in the art can design for any wavelength rangein the electromagnetic spectrum. The dashed curve 351 in FIG. 4 crepresents the dispersion of the photonic crystal slot waveguide guidedmode.

The left image in FIG. 4 d represents one possible embodiment of thephotonic crystal slot waveguide section 102 where all columnar members103 adjacent to the photonic crystal slot waveguide 102 on both sides ofthe photonic crystal slot waveguide 102 are separated by the latticeconstant α of the periodic lattice. However, each row of columnarmembers 103 parallel to the photonic crystal slot waveguide 102 can beshifted in either the x-direction or the z-direction in the plane of theslab. The same shift is made for each row of columnar members 103 on theopposite side of the photonic crystal slot waveguide 102, and the samenumber of lattice periods perpendicularly away from the photonic crystalslot waveguide 102. For example, x1 denotes the shift of the first rowof columnar members 103 that are adjacent to the photonic crystal slotwaveguide 102, on both sides of the photonic crystal slot waveguide 102,in the x-direction as indicated by the coordinate system arrow to theleft of the figures. Positive x values and positive z values indicatethat the shift of the columnar members is made in the direction of thecorresponding coordinate system arrow. Negative x values and negative zvalues indicates that the shift of the columnar member or members ismade in the direction opposite to the direction of the correspondingcoordinate system arrow in FIG. 4 d. A zero value for x or z indicatesno shift of the corresponding columnar members. The arrows for each ofx1, x2, x3, z1, z2, and z3 indicate the direction of the shift. Theright image in FIG. 4 d represents a fabrication tolerant geometry wherex1=0, x2=−0.15α, x3=0.3α, absolute value of z1=0.2α, absolute value ofz2=0.05α, and z3=0. When absolute value of z1=0.2α, the width of 141changes from 1 times sqrt(3) times a in the left image in FIG. 4 d to1.4 times sqrt(3) times a in the right image of FIG. 4 d.

The solid curve 361 in FIG. 4 e represents the group index profilecalculated from the dispersion diagram similar to FIG. 4 c, for the leftimage in FIG. 4 d. The group index is inversely proportional to thegroup velocity. Since the absorbance is dependent upon the slow-down oflight, the group index determines the magnitude of slow down at aparticular wavelength. A constant group index implies that the devicewill have a constant absorption efficiency. Since the group index isdetermined by the device fabrication parameters, which include thediameter of the columnar members 103 and the slot 101, the group indexcurve 361 can shift in wavelength if the diameter of the fabricatedcolumnar members 103 and the dimension of the slot 101 are differentfrom designed group index curve 361 as a function of wavelength as shownin FIG. 4 e. If the target group index is 27 as in FIG. 4 e, thewavelength bandwidth is only 7 nm over which the group index is within10% of the target group index. We consider that the target wavelength atwhich the absorbance of the analyte is measured is 9500 nanometers (nm).However, fabrication differences in the diameter of the columnar members103 and the slot 101 can alter the group index curve position withrespect to wavelength and hence the absorbance, which is a directfunction of group index, as shown in Equation 2. Hence the fabricationtolerant design of the right image of FIG. 4 d is designed with theindicated shifts so that the group index curve is flat, in other words,the group index is substantially constant, over a wide or extendedbandwidth. The wavelength bandwidth is determined by the range ofwavelength over which the group index is within 10% of the target groupindex. The wavelength bandwidth in the constant group index design is 60nm, as shown by the curve 362. Hence the design of the right image ofFIG. 4 d represents a fabrication tolerant design for the specificwavelength of 9500 nm for a particular choice of the semiconductor slab104 and the bottom cladding 105 on the substrate 107 and the choice ofmaterial, whether air or hydrophobic polymer which fills the columnarmembers 103 and the slot 101. The flat group index profile is afabrication tolerant group index profile that can be designed for anywavelength of operation as determined by the absorbance peak of theanalyte that is to be detected. The wide wavelength bandwidth constantgroup index profile can be achieved for different values of x1, x2, x3,z1, z2, and z3 each of which can vary continuously from −0.5 α to 0.5 αwhere α is the lattice constant, depending upon the dielectric constantor refractive index of the semiconductor slab 104, bottom cladding 105,substrate 107 and the material, air or hydrophobic polymer which fillsthe columnar members 103 and slot 101.

With a fabrication tolerant group index profile, as indicated by thecurve 362 in FIG. 4 e, the input and output impedance taper sections 115and 114 respectively also need to have a gradual change in group indexas indicated by FIG. 4 f by the series of group index plots 371-386. Thegradual change in group index for the series of plots 371-386 isachieved by progressively moving the first, second, and third rows ofcolumnar member 103 similar to the right image in FIG. 4 d in both x-and z-directions on both sides of the photonic crystal slot waveguideonly in the input and output impedance taper sections 115 and 114. Arepresentative situation is indicated in FIG. 4 g for which the groupindex plots 371-386 were obtained in FIG. 4 f. The input impedance tapersection is indicated by the section between the dashed line H-H′ and thedashed line I-I′, with the series of z1 values ranging from z1 ₁ to z1₁₆ as indicated in FIG. 4 g. The section of FIG. 4 g to the right sideof the dashed line H-H′ is the photonic crystal slot waveguide regionindicated by the right image in FIG. 4 d. The photonic crystal slotwaveguide region to the right side of the dashed line H-H′ in FIG. 4 gextends to the output impedance taper section as indicated in FIG. 1.The position of the columnar members 103 to the right side of the dashedline H-H′ in FIG. 4 g, on the first, second, and third rows on bothsides of the photonic crystal waveguide are kept constant at the valuesindicated in the right image in FIG. 4 d to achieve the group indexprofile 362 in FIG. 4 f. In the input impedance taper section betweenthe dashed lines H-H′ and I-I′, the values of x1, x2, x3, z2, and z3 onboth sides of the photonic crystal slot waveguide, are given by: x1=0,x2=−0.15α, x3=0.3α, absolute value of z2=0.05α, and z3=0 where a denotesthe lattice constant of the periodic photonic crystal pattern. In thephotonic crystal slot waveguide region to the right side of the dashedline H-H′, values for x1, x2, x3, z1, z2, and z3 have been shown only onone side of the photonic crystal slot waveguide for clarity. The samevalues of x1, x2, x3, z1, z2, and z3 hold on the other side of thephotonic crystal slot waveguide as has been shown in the right image inFIG. 4 d. The values of z1 in the input impedance taper section are asindicated in the figure from z1 ₁ to z1 ₁₆ over 16 lattice periods ofthe photonic crystal pattern, where z1 ₅ for instance denotes the 5^(th)lattice period from the dashed line H-H′. Note that p equals 16 in FIG.4 g. The design of the output impedance taper section is exactly thesame as the design of the input impedance taper section described inFIG. 4 g. One skilled in the art will note that the progressive changein group index plots 371-386 can also be achieved by a progressivechange in the radius of the columnar members in the input and outputimpedance taper sections.

The bold curve 201 in FIG. 5 a depicts the transmission spectrum of thephotonic crystal slot waveguide device in the absence of methane gas asa representative analyte, over the wavelength range from 1663 nm to1679.5 nm. Corresponding to the slow light region or low group velocityregion 162 in the dispersion diagram in FIG. 4 b, the slow light regionor region of low group velocity in the transmission spectrum is shown as172 in FIG. 5 a. The sharp drop in the transmission spectrum at about1679 nm corresponds to the band edge of the photonic crystal slotwaveguide. The bold curve 202 in FIG. 5 b depicts the transmissionspectrum of the photonic crystal slot waveguide device in the presenceof methane gas as a representative analyte, over the wavelength rangefrom 1663 nm to 1679.5 nm. Corresponding to the slow light region orregion of low group velocity region 162 in the dispersion diagram inFIG. 4 b, the slow light region or region of low group velocity in thetransmission spectrum is shown as 172 in FIG. 5 b. The sharp drop in thetransmission spectrum at about 1679 nm corresponds to the band edge ofthe photonic crystal slot waveguide. The bold curve 203 in FIG. 5 cdepicts the difference of the transmission spectra 201 and 202 of thephotonic crystal slot waveguide device in the absence and presence ofmethane gas respectively, that determines the absorbance of methane overthe wavelength range 1663 nm to 1675 nm. The spectrum of FIG. 5 c showsvery good correspondence with the theoretically calculated spectrum ofmethane in the range of 1666 nm [HITRAN spectroscopic database, Rothmanet al., “The HITRAN 2008 molecular spectroscopic database,” J.Quantitative Spectroscopy and Radiative Transfer 110, 533 (2009)].However, one skilled in the art will note that the analyte can be anysolid, liquid, or gas and the range of wavelength can be any range inthe entire electromagnetic spectrum (ultraviolet, visible, infrared(near-, mid- and far-)), which displays absorption signatures of thatcorresponding specific solid, liquid, or gas.

In one embodiment, the slab 104 is formed from a material of highrefractive index including, but not limited to, silicon, germanium,carbon, gallium nitride, gallium arsenide, gallium phosphide, indiumnitride, indium phosphide, indium arsenide, zinc oxide, zinc sulfide,silicon oxide, silicon nitride, alloys thereof, metals, and organicpolymer composites. Single crystalline, polycrystalline, amorphous, andother forms of silicon may be used as appropriate. Organic materialswith embedded inorganic particles, particularly metal particles, may beused to advantage. In one embodiment, the top cladding 106 and bottomcladding 105 are formed from a material whose refractive index is lowerthan that of the slab material. Suitable top cladding and bottomcladding materials include, but not limited to, air, silicon dioxide,silicon nitride, alumina, organic polymers and alloys thereof. Thesubstrate 107 materials include, but not limited to, silicon, galliumarsenide, indium phosphide, gallium nitride, sapphire, glass, polymerand alloys thereof. In one embodiment, the columnar members 103 areformed from a material whose refractive index is substantially differentfrom that of the slab 104. Suitable materials for the columnar members103 include, but not limited to, air, silicon oxide, silicon nitride,alumina, organic polymers, or alloys thereof. In one preferredembodiment, the slab 104 is formed from silicon, the columnar members103 are formed from air, the top cladding 106 is air, and the bottomcladding 105 is formed from silicon dioxide, while the substrate 107 issilicon.

Although the word “analyte” is used in the preceding discussions, oneskilled in the art will understand that it refers to a general form ofanalyte that includes solids, liquids, and gases.

Although the word “light” or “lightwave” is used to denote signals inthe preceding discussions, one skilled in the art will understand thatit refers to a general form of electromagnetic radiation that includes,and is not limited to, visible light, infrared light, ultra-violetlight, radio waves, and microwaves.

In summary, the present invention provides an ultra-compact on-chipspectrometer using a two-dimensional photonic crystal slot waveguide.The invention will lay a solid foundation for the future of on-chipspectroscopy devices for label free massively parallel detection andidentification of analytes in lab-on-chip applications. Owing to thesmall dimensions of the devices presented herein, one can monolithicallyintegrate the photonic crystal slot waveguide spectrometers on siliconVLSI chips. The CMOS compatible photonic crystal slot waveguide deviceshave simpler design requirements than the microelectronics industry.Furthermore, easy regeneration capability, less complicated optics,on-chip slot waveguide analyte sampling cell, and on-chip integrationensures that our miniature compact devices will deliver improved resultswith significantly lower cost to the customer. The device will enablelow cost on-chip chemical and biological analysis and diverseapplications in industrial process control and monitoring, homelandsecurity, and environmental pollution monitoring.

While the invention has been described in connection with a number ofpreferred embodiments, it is not intended to limit the scope of theinvention to the particular form set forth, but on the contrary, it isintended to cover such alternatives, modifications, and equivalents asmay be included within the design concept of the invention as defined bythe appended claims.

1. A method for on-chip optical absorption spectroscopy, the methodcomprising: generating electromagnetic radiation from a broadbandsource; coupling the electromagnetic radiation from the broadband sourceto an on-chip optical absorption spectroscopy apparatus comprising: i) asubstrate; ii) a bottom cladding disposed on the substrate; iii) a slabdisposed on the bottom cladding, wherein the refractive index of thebottom cladding is lower than the refractive index of the slab; iv) aplurality of void columnar members etched through the slab, wherein theplurality of void columnar members form a periodic lattice with alattice constant α; v) a core in the slab comprising an input side andan output side, wherein the core is free of void columnar members fromthe input side to the output side; vi) wherein the distance betweenimmediately adjacent inter-row void columnar members between a first rowof void columnar members of the lattice from the input side to theoutput side immediately adjacent both sides of the core, a second row ofvoid columnar members of the lattice from the input side to the outputside immediately adjacent the first row of void columnar members of thelattice, and a third row of void columnar members of the lattice fromthe input side to the output side immediately adjacent the second row ofvoid columnar members of the lattice is not equal to the latticeconstant α; vii) a photonic crystal slot waveguide comprising: a) aphotonic crystal waveguide formed within the core; b) one or more slotwaveguides defined by one or more void slots within the photonic crystalwaveguide, extending the entire length of the photonic crystalwaveguide; c) an input impedance taper in the photonic crystal waveguideat the input side; and d) an output impedance taper in the photoniccrystal waveguide at the output side; viii) an input ridge waveguidebeyond the photonic crystal waveguide coupled to the input impedancetaper in the photonic crystal waveguide; ix) an output ridge waveguidebeyond the photonic crystal waveguide coupled to the output impedancetaper in the photonic crystal waveguide; x) wherein the one or more voidslots within the photonic crystal slot waveguide extend into the inputridge waveguide and the output ridge waveguide; xi) wherein the photoniccrystal slot waveguide supports one or more guided modes of thebroadband source; and xii) wherein an output transmission spectrumintensity of the one or more guided modes of the broadband source variesas a function of the absorbance of an analyte within the photoniccrystal slot waveguide; measuring a first output transmission spectrum,wherein measuring the first output transmission spectrum comprisesmeasuring the electromagnetic radiation from the output ridge waveguideof the on-chip optical absorption spectroscopy apparatus in the absenceof analyte within the photonic crystal slot waveguide; exposing theon-chip optical absorption spectroscopy apparatus to the analyte;measuring a second output transmission spectrum, wherein measuring thesecond output transmission spectrum comprises measuring theelectromagnetic radiation from the output ridge waveguide of the on-chipoptical absorption spectroscopy apparatus in the presence of analytewithin the photonic crystal slot waveguide; taking the differencebetween the first output transmission spectrum and the second outputtransmission spectrum to determine an absorbance spectrum; andidentifying the analyte from the absorbance spectrum.
 2. The method ofclaim 1, further comprising measuring a magnitude of the absorbancespectrum; and determining a concentration of the analyte from themeasured magnitude of the absorbance spectrum.
 3. The method of claim 1,wherein the periodic lattice comprises a triangular lattice or a squarelattice.
 4. The method of claim 1, wherein the on-chip opticalabsorption spectroscopy apparatus further comprises: a superstratedisposed on the slab, wherein the refractive index of the superstrate islower than the refractive index of the slab.
 5. The method of claim 4,wherein the superstrate, the one or more void slots, and the pluralityof void columnar members are filled with material selected from thegroup consisting of: air, silicon dioxide, silicon nitride, andhydrophobic polymer that selectively absorbs the analyte from water. 6.The method of claim 1, wherein the input impedance taper comprises firstp void columnar members immediately adjacent both sides of the core fromthe input side separated by the width of the core plus 2x times α, wherex ranges from z to 0 in decrements of (x times α)/p, where p is a numbergreater than 4 and z is a fractional number between 0 and 1; and whereinthe output impedance taper comprises first p void columnar membersimmediately adjacent both sides of the core from the output sideseparated by the width of the core plus 2x times α, where x ranges fromz to 0 in decrements of (x times α)/p, where p is a number greater than4 and z is a fractional number between 0 and
 1. 7. The method of claim1, wherein the input impedance taper comprises first p void columnarmembers immediately adjacent both sides of the core from the input sideseparated by the width of the core plus 2x times α, where x ranges fromz to 0 in decrements of (x times α)/p, where p is a number greater than4 and z is a fractional number between −1 and 0; and wherein the outputimpedance taper comprises first p void columnar members immediatelyadjacent both sides of the core from the output side separated by thewidth of the core plus 2x times α, where x ranges from z to 0 indecrements of (x times α)/p, where p is a number greater than 4 and z isa fractional number between −1 and
 0. 8. The method of claim 1, whereinthe material of the slab is selected from the group consisting of:silicon, germanium, carbon, gallium nitride, gallium arsenide, galliumphosphide, indium nitride, indium phosphide, indium arsenide, zincoxide, silicon oxide, silicon nitride, alloys thereof, and organicpolymers.
 9. The method of claim 1, wherein the plurality of voidcolumnar members is filled with material selected from the groupconsisting of: air, silicon oxide, silicon nitride, and organicpolymers.
 10. The method of claim 1, wherein the analyte is disposedwithin the one or more void slots and/or one or more void columnarmembers in close proximity to the photonic crystal slot waveguide,wherein the one or more void slots and the one or more void columnarmembers in close proximity to the photonic crystal slot waveguide arefilled with material selected from the group consisting of: air andhydrophobic polymer that selectively absorbs the analyte from water. 11.The method of claim 1, wherein the effective absorbance of the one ormore guided modes of the broadband source by the analyte is directlyproportional to the enhancement of the electric field intensity of theone or more guided modes of the broadband source.
 12. The method ofclaim 1, wherein the effective absorbance of the one or more guidedmodes of the broadband source by the analyte is inversely proportionalto the group velocity of the one or more guided modes of the broadbandsource.
 13. The method of claim 1, wherein the on-chip opticalabsorption spectroscopy apparatus further comprises a filter configuredto filter out particles from the analyte and wherein the filtercomprises an on-chip microfluidic filter or an off-chip macroscopicfilter.
 14. The method of claim 1, wherein the on-chip opticalabsorption spectroscopy apparatus further comprises a filter configuredto absorb the moisture of the analyte.
 15. The method of claim 1,wherein the broadband source ranges from ultraviolet to far infrared.16. The method of claim 1, wherein the on-chip optical absorptionspectroscopy apparatus further comprises: an on-chip light emitterconfigured to emit electromagnetic radiation of the broadband sourceinto the core; and an on-chip light detector configured to detect theoutput transmission spectrum intensity of the one or more guided modesof the broadband source.
 17. The method of claim 16, wherein the on-chiplight emitter comprises a light emitting diode or a semiconductor laserand the on-chip light detector comprises a photodiode.
 18. The method ofclaim 16, wherein the on-chip light emitter comprises a frequencymodulated on-chip light emitter or a wavelength modulated on-chip lightemitter.